This invention is in the field of low elevationangle radar tracking, and specifically addressed to a system that enables such tracking to be accurately obtained.
One of the major requirements for radar systems including ship-based, land-based and airborne radar system, is to have a tracking system, which may be automatically controlled, for low angle targets such as aircraft.
Because of the earth's curvature, target aircraft are generally of the low-flying short to medium range aircraft, and medium to high flying aircraft at greater ranges.
To understand the problems encountered in the prior art, a brief description of angular elevation tracking heretofore utilized is necessary.
IN A PENCIL BEAM ELECTRONIC SCANNED RADAR SYSTEM, THE BASIC APPROACH USED FOR TRACKING TARGETS IN EITHER AZIMUTH OR ELEVATION ANGLES IS TO OBTAIN TWO MEASURES OF THE TARGET'S POSITION, "BEAM-SPLIT" THE RESULTANT MEASURES FOR ACCURACY, AND THEN FEED THE BEAMSPLIT INFORMATION TO A TRACK SMOOTHING COMPUTER ROUTINE WHICH TAKES OUT ANY SHORT TERM TRANSIENT MEASUREMENT ERRORS AND PRODUCES A SMOOTH PLOT OF THE TARGET'S ACTUAL POSITION. The measures are obtained by either a sequential lobing technique, that is firing two beams, one on each side of the expected predicted target position, or by a monopulse technique wherein two apparent receive beams equally spaced about the transmit beam position are formed. In the former case, the difference in the response of the two transmit beams is then used to provide a measure of how far from the predicted position the actual target is located. For example, the amplitude response of one beam will be greater than that of the other beam since the actual target may be closer to the position of the peak of said one beam, that is, close to the nose of such beam.
However, some of the problems encountered in any of the systems heretofore used were in prior attempts to permit low angle tracking, substantially unsuccessful. Such prior attempts ignored the problem of tracking long range targets and relied on an operator to track reacquisitions of short range targets. Though it may be possible to track short range targets in view of such targets being visible on a search PPI display, it is obviously both ineffective as well as inefficient when more than a few targets are simultaneously present.
Further, the following specific problems prevail in prior art tracking:
Low angle tracks beyond the clutter processing range, that is targets at long range, would get pulled off the target and follow clutter. This would often lead to track splitting situations and hence an observer would lose control of a tracking situation because track symbols would literally be running wild across the display face.
Tracks of low angle medium to long range targets would become lost. They would be pulled off the target track due to multipath effects which produced inaccurate amplitude responses from the beams. This inaccuracy is especially deleterious in a sequential lobing situation, such as is used in a typical radar system, since the two beams are fired at different times and hence have different multipath amplitude distortions.
Low angle short range tracks would be lost due to distortion of the transmit-receive beams. This is caused by part of the beam pattern to be directly in line with the ground or sea and create distortion due to the lower elevation portion of the beam interferring with the ground or sea surface. A distorted beam will not provide true amplitude response and hence the resultant beam splitting would indicate a wrong position for the target.
In order to allow the radar system to operate so as not to be troubled by the above problems, a "fix" was incorporated, which merely dropped all low angle (less than 0.3.degree. ) tracks. This, of course, was not a remedy for insofar as acquisition or tracking of low angle targets, but was rather an avoidance of the problems inherent to the system at that time.
Hence the system heretofore employed was one that did not permit successful tracking of low elevation targets.
Several papers given on this subject also attempted unsuccessfully to resolve the above-stated problem as to low angular targets, at the Proceedings of the International Conference at London, England on Oct. 23-25, 1973.
Paper A74-12376, entitled Accurate Tracking of Low Elevation Targets Over the Sea with Monopulse Radar was published in the publication of English Institution of Electrical Engineers, 1973, pages 160-165. This publication relates to a monopulse radar tracking of targets whose elevation is less than the radar beamwidth above a sea surface. Such tracking becomes unreliable since the radar system tries to null on a composite signal of the target and its image. It is shown that there is a continuous locus equilibrium positions which the radar may follow, as well as isolated regions of equilibrium about the image which can cause loss of tracking. Various schemes are discussed for maintaining track and obtaining elevation data on a low-flying target. The solution however requires three independent beams and hence a complex radar system.
Paper A74-12374, entitled The Low-Angle Tracking Problem, was published in the English publication of the Institution of Electrical Engineers in 1973, pages 146-153. This publication relates to the amplitude and angularspread characteristics of radar reflections from earth surface irregularities, examined on the basis of data obtained from communications experiments. The basic phenomena of surface relfection are shown to produce irregular and unpredictable components as well as the specular (image) reflections. The effects of such on radar tracking are evaluated by considering the combination of the target and reflection signals as a glinting target extended in azimuth and elevation but occupying the same range resolution cell as the desired target.
Paper A74-12377, entitled Multi-Frequency Complex Angle Tracking of Low Level Targets, was published in the English publication of The Institute of Electrical Engineers in 1973, pages 166-171. This publication states that radar tracking of a low-flying target becomes unsatisfactory when the target is at an elevation of less than about two thirds of the radar beamwidth above a fairly smooth surface. This is due to `glint` between the target and its image, which merge into an unresolved double target. The relative sum and difference signals can be processed to yield a complex indicated angle, but this complex angle cannot be used to determine the real angle uniquely without more information. The present paper describes a method for determination of the real angle by using complex angle measurements at more than one frequency. Digital computer simulation studies of the method indicate that it provides considerable improvement in low-level tracking over a smooth surface, but marginal improvement over rough surfaces. The limiting factor is frequently diffuse reflection from a rough surface, and it appears that this will also limit other methods of implementing complex-angle tracking.
It may be seen from the foregoing that the prior art radars and studies thereof have not provided accurate low elevation and angle target tracking.
In an attempt to overcome the problem of tracking multiple fast moving and low flying targets, a computer program was utilized in conjunction with the radar system. Though it was possible to track at lower elevation angles than heretofore with use of computer software, when used with the low angle mode for single beam tracking, a high false alarm rate of prevailing targets resulted.